1. Field of the Invention
The present invention generally relates to the processing of image signals supplied from a CMOS image sensor for image pickup, and particularly relates to the correction of defect pixels contained in the image signals from a CMOS image sensor.
2. Description of the Related Art
A CMOS image sensor includes a reception section in which a plurality of photo-diodes are arranged in a matrix array. These photo-diodes correspond to individual pixels for the imaging purpose. Incident light is subjected to photoelectric conversion on a pixel-by-pixel basis, and the electric charge obtained by the photoelectric conversion is accumulated in an electric charge storage section for reading of data. A large number of pixels inevitably include defective pixels that do not operate properly, which appear as fixed dark dots (black dots) or bright dots (white dots)
FIG. 1 is a block diagram showing the construction of a unit for defect correction of a CMOS image sensor.
The unit of FIG. 1 includes a defect correcting unit 11, a defect position storage device 12, and a RGB conversion unit 13. The defect correcting unit 11 receives image signals from an image sensor. The defect correcting unit 11 processes the data of defect pixels of the image signals for correction of defects, and supplies the corrected image signals to the RGB conversion unit 13. The RGB conversion unit 13 obtains color data of each pixel based on color information of the RGB Bayer array.
The unit for defect correction as shown in FIG. 1 is provided with the defect position storage device 12, which is a ROM (read only memory) for storing defect positions. The defect position storage device 12 has information recorded therein indicating the position of pixels that are ascertained as defective by the test conducted just prior to shipment. The defect correcting unit 11 identifies the position of defect pixels by referring to the position information recorded in the defect position storage device 12. Using information about surrounding pixels, the defect position storage device 12 performs interpolation so as to correct the data of each defect pixel.
In this manner, defect pixels appearing as dark (black) dots or bright (white) dots are interpolated by use of the surrounding pixels for blending into the surrounding image.
Japanese Patent Application Publication No. 2001-307079 discloses a method of correcting the electronic data of defect pixels with high accuracy. This method selects a pair of pixels having the smallest difference from the pairs of pixels surrounding a defect pixel, and interpolates a value of the defect pixel by use of an average of the two pixel values of the selected pixel pair.
Defects occur with certain probability (e.g., in accordance with the rate of defects generated during a wafer process). When a ROM is used as described above, correction may not be possible due to the limited storage size of the ROM if the number of defects exceeds the expected maximum number. Further, the ROM needs to store the information indicative of the position of pixels that are ascertained as defective by the test prior to shipment. If defects occur after the test as a result of deterioration with time, such defects cannot be corrected.
Accordingly, t here is a need for a defect correcting circuit which can cope with any number of defects, and can cope with such defects as occurring as a result of deterioration with time.